Constraining the quadrupole moment of stellar-mass black-hole candidates with the continuum fitting method

TL;DR

This paper demonstrates that current X-ray observations of accretion disks can be used to constrain the quadrupole moments of stellar-mass black hole candidates, providing a test of their Kerr nature in General Relativity.

Contribution

It introduces a method to constrain black hole quadrupole moments using existing X-ray data, enabling tests of the Kerr hypothesis without future gravitational-wave measurements.

Findings

Constraints on quadrupole moments derived from X-ray data.

Method to test black hole Kerr nature with current observations.

Potential to confirm or challenge General Relativity predictions.

Abstract

Black holes in General Relativity are known as Kerr black holes and are characterized solely by two parameters, the mass $M$ and the spin $J$. All the higher multipole moments of the gravitational field are functions of these two parameters. For instance, the quadrupole moment is $Q=-J^2/M$, which implies that a measurement of $M$, $J$, and $Q$ for black hole candidates would allow one to test whether these objects are really black holes as described by General Relativity. While future gravitational-wave experiments will be able to test the Kerr nature of these objects with very high accuracy, in this paper we show that it is possible to put constraints on the quadrupole moment of stellar-mass black hole candidates by using presently available X-ray data of the thermal spectrum of their accretion disk.